Method of operating a multi-roll casting machine during and after freezing of the liquid core of the strand

ABSTRACT

A method of and apparatus for operating a multiple-roll machine, especially a multiple-roll withdrawing machine or a multiple-roll combination withdrawing and straightening machine in a continuous casting plant, wherein a casting and a dummy bar are passed between pairs of rolls. By means of each pair of rolls there is applied to the casting and the dummy bar as they pass therebetween, the requisite contact pressure for generating the necessary traction or withdrawal force. When the thickness of the casting falls below a prescribed thickness there is applied to the casting only a contact pressure of a magnitude required for traction and when the thickness of the casting exceeds the prescribed thickness the contact pressure is increased to a greater value calculated to again reduce the thickness of the casting to the prescribed thickness.

United States Patent [1 1 Bollig et al.

[111 3,812,900 [4 1 May28, 1974 [54] METHOD OF OPERATING A MULTI-ROLL CASTING MACHINE DURING AND AFTER FREEZING OF THE LIQUID CORE OF THE STRAND [75] Inventors: Georg Bollig, Buderich; Horst Grothe, Kaarst, both of Germany; Bernhard Knell, Thalwil, Switzerland; Armin Thalmann; Josef Zeller, both of Weesen, Switzerland [22] Filed: June 29, 1972 [21] Appl. No.: 267,531

[30] Foreign Application Priority Data July 3,1971 Germany 2133144 [52] US. Cl 164/76, 164/270, 164/282 [51] Int. Cl 822d 11/12 [58] Field of Search 164/4, 76, 82, 270, 282

[56] References Cited UNITED STATES PATENTS Harter, Jr. et al. 164/282 X Orr et a1. 164/154 Primary ExaminerR. Spencer Annear Attorney, Agent, or Firm-Werner W. Kleeman 5 7 ABSTRACT A method of and apparatus for operating a multiple roll machine, especially a multiple-roll withdrawing machine or a multiple-roll combination withdrawing and straightening machine in a continuous casting plant, wherein a casting and a dummy bar are passed between pairs of rolls. By means of each pair of rolls there is applied to the casting and the dummy bar as they pass therebetween, the requisite contact pressure for generating the necessary traction or withdrawal force. When the thickness of the casting falls below a prescribed thickness there is applied to the casting only a contact pressure of a magnitude required for traction and when the thickness of the casting exceeds the prescribed thickness the contact pressure is increased to a greater value calculated to again reduce the thickness of the casting to the prescribed thickness.

4 Claims, 5 Drawing Figures METHOD OF OPERATING A MULTI-ROLL CASTING MACHINE DURING AND AFTER FREEZING OF THE LIQUID CORE OF THE STRAND BACKGROUND OF THE INVENTION The present invention relates to a new and improved method of withdrawing a casting during and after the liquid core has frozen and feeding a dummy bar in a multiple-roll withdrawing machine or a multiple roll withdrawing and straightening machine in a continuous casting plant for steel equipped with an at least partly arched apron, which contemplates applying to the casting and the dummy bar as they pass between the pairs of driving and/or straightening rolls, by means of each pair of rolls, the requisite Contact pressure for generating the necessary traction. This invention also concerns apparatus for performing the aforesaid method.

The withdrawing machine in a continuous casting plant withdraws the casting by frictionally engaging same, and a withdrawing and straightening machine also straightens the arched or curved casting. In many continuous casting installations the withdrawing machine is also used to feed the dummy bar back to the mold. In order to enable the driving rolls to apply the traction or braking force to the casting and the dummy bar, these rolls must contact the casting and the dummy bar with the requisite amount of pressure. These contact pressures depend upon the required tractive or braking effort, the coefficient of friction between the casting or dummy bar and each of the driving rolls as well as upon the number of driving rolls present. In the case of castings still having a liquid core when they enter the withdrawing and straightening machine the contact pressure of the driving rolls depends upon the ferrostatic pressure and must be sufficient to prevent the casting from bulging. The contact pressure needed for straightening the casting considerably depends upon the temperature of the casting. With such castings the contact pressure required for straightening is less than the pressure needed to prevent bulging.

In a continuous casting plant for steel the straightening and driving rolls of so-called multiple roll withdrawing and straightening machines are distributed along a major portion of the path of the casting. At least some of the straightening and driving rolls may make contact with the casting where such still has a liquid core. Ac-

' cording to existing casting parameters, such as pouring speed, pouring temperature and so forth, the temperature of the casting will also vary as well as the position of the apex of the liquid core. In the region where the apex of the liquid core freezes the ferrostatic pressure on the frozen shell will disappear, but the casting still is very plastic and easily deformable. For instance, a temperature range of a slab between 900C and 1,0'50C in the withdrawing and straightening machine corresponds to a range of its deformation resistance'between about 3 and 7 kg/mm The contact pressures which prevent bulging of castings still having a liquid core are then quite sufficient to cause a significant reduction in thickness by about 2 to ID mm. in the zone where the liquid core of a slab has just frozen. When using automatic casting cutters or croppers which cutoff preadjusted lengths such reductions in thickness result in billets of unequal weights which have undesirable effects during subsequent processing. Moreover, squeezing of the casting between the rolls during its solidification may also give rise to defects in the casting structure.

It is a well known practice to use a multiple-roll withdrawing and straightening machine to withdraw and straighten a casting having a liquid core as well as to feed a standard dummy bar having a substantially thinner cross-section than the casting. The straightening rolls are forced hydraulically against fixed stops by piston-and-cylinder units to prevent the casting having a liquid core from being undesirably rolled-down as it passes between these straightening rolls. Contrary to the straightening rolls the contact pressure of the hydraulically loaded driving rolls is adjusted by remotely controlled valve means according to the resistance of the growing frozen shell and the change in ferrostatic pressure. In such a multiple-roll withdrawing and straightening machine the contact pressure of the driving rolls is so controlled that it is only 15% higher than the pressure that would be needed to prevent bulging. It is thus possible to avoid bulges from appearing on a casting having a liquid core. In the region where the liquid core finally freezes, i.e. where the ferrostatic pressure vanishes, the driving rolls of this machine are responsible for the above-described undesirable uncontrolled reductions of the casting below the prescribed thickness as well as unwanted internal flaws in the form of cracks in the horizontal casting axis.

Another withdrawing and straightening machine known in the art for a continuous steel casting plant has separate sections each comprising three upper and three bottom rolls. The three upper rolls are mounted in a yoke which is tiltable roughly about the axis of the center roll and adjustably movable in a direction normal to the guided surface of the casting. The middle roll of the bottom set is mounted in fixed bearings in the supporting framework, whereas the two outer bottom rolls are yieldingly mounted on thrust cylinders. In order to control the ferrostatic pressure of the liquid core and to prevent bulging this machine also requires the application of contact pressures by both the upper rolls and by the yieldingly mounted lower rolls. In such withdrawing and straightening machines an undesirable reduction of the required thickness of the steel casting immediately after the solidification of the liquid core can therefore also occur.

SUMMARY OF THE INVENTION Therefore it is an object of the present invention to provide an improved method of and apparatus for performing the aforesaid method, which will permit a casting to be prevented from bulging and also from experiencing an undesirable reduction in thickness below a prescribed thickness and from developing internal.

flaws as it passes through a multiple-roll withdrawing and/or straightening machine, irrespective whether the part of the casting passing through the machine still contains a liquid core or whether this has just frozen, i.e. solidified. The machine is also intended to feed a dummy bar to the mold and to again withdraw same.

Another object of the invention is the provision of a withdrawing and/or straightening machine having new and improved means for controlling the desired thickness of a casting and to provide adjustments necessary for preventing any undesirable reduction in the casting thickness.

The proposed method achieves these objects by applying only the contact pressure required for traction or feeding when the thickness of the casting falls below the prescribed thickness and raising the contact pressure to a higher value calculated to reduce the casting to its prescribed thickness when this is exceeded.

The apparatus for performing this method contemplates that at least one roll of each pair of rolls is movable by additional thrustors or force-applying devices towards the casting beyond a stop defining the prescribed thickness, but that the thrust or contact force generated by said thrustors is smaller than that generated by the cylinder-and-piston units.

The proposed method enables the portion of a casting in which the liquid core is still present or in which the liquid core has just frozen to be merely withdrawn or to be withdrawn and straightened in multiple-roll withdrawing or in multiple-roll withdrawing and straightening machines practically without changing its predetermined thickness and irrespective of any changes that may occur in the casting parameters. Tolerances can be maintained within 1 percent of the prescribed thickness in such a casting. in heats used for the production of plate the method according to the invention also permits avoiding internal flaws caused by undesirable roll squeezing. Furthermore, dummy bars or standard dummy bars having cross-sections that are smaller than the thicknesses of the castings by any amount can be readily fed to the mold and withdrawn.

If at least two rolls that are consecutive in the direction of travel of the casting are mounted in a common yoke which divides a common thrust or contact force between them, then their respective contact pressures on the casting will be the same, irrespective of the tilt of the yoke. For example, because of a disturbance preceding or inside the withdrawing machine it is possible for a casting to bulge between the two rolls so that a surface of limited extent exceeds the prescribed dimensions. Moreover, when stoppages occur during a pour excessive local cooling may cause the casting at certain points to fall short of the prescribed thickness because of shrinkage. ln a two-roll yoke one roll may assume a position which is within that corresponding to the prescribed thickness of the casting, whereas the other may be located in a position outside this latter position. The roll located within the prescribed thickness of the casting would then roll down the casting to an even smaller thickness. If the dimensional tolerances in the thickness of the casting are to be minimized another proposal according to the invention therefore provides for a comparison to be made between the actual positions of the outer rolls and the reference positions the rolls should occupy if the thickness of the casting were as prescribed. Upon generation ofa deviation signal from the outer roll which is outside the prescribed thickness, and in the absence of a deviation signal from the other outer roll, or upon the appearance of a deviation signal from one of the outer rolls which is outside the prescribed thickness by a given amount and of a second deviation signal from the other outer roll located within the prescribed thickness by the same or an even greater amount, it is proposed to reduce the contact pressure of the other roll, irrespective of the contact pressure of the one roll.

The proposed apparatus for performing this method comprising rolls mounted on yokes is manifested by the association with the yoke near the bearings of the outer rolls of position reporting or transmitting devices for signalling the position of the rolls in relation to their positions corresponding to the prescribed thickness of the casting, and by the association with each roll of independently controllable supporting means which oppose the contact pressure exerted by the rolls.

The driving rolls may apply the same contact pressure to a casting which is thinner than the prescribed thickness as to a dummy bar which is also thinner than the prescribed casting thickness. Moreover, for the purpose of further reducing the contact pressure needed for merely withdrawing the casting different contact pressures may be applied to a casting which is thinner than the prescribed thickness and to a dummy bar which is thinner than the prescribed thickness of a casting by taking advantage of the differential coefficients of friction between the driving rolls and the casting respectively the dummy bar.

An advantageous embodiment of the proposed apparatus consists in mounting the thrustors on the piston rods of the piston-and-cylinder units.

The thrustors may be inexpensively and simply constituted by stacks of Belloile or cup springs. However, in order to permit the pressure generating force to be quickly and precisely adjusted when the width of the casting changes, it is preferred to embody the thrustors in hydraulic piston-and-cylinder units.

To enable adjustments for changes in cross-section affecting the thickness of the casting to be quickly and accurately made, another feature of the invention consists in providing exchangeable abutment or intermediate members between the stops on the piston rods and the associated cylinders for suitably varying the gap width between the rolls which determines the prescribed thickness of the casting.

The bottom driving and straightening rolls may be firmly supported by the structure of the machine and only the upper rolls arranged to be movable perpendicular to the casting surface. In the case of cold castings which cannot be precisely straightened it is proposed to insure the presence of the necessary thrust by the provision of buffer members. According to another aspect of the invention these buffer members may be differential thrustors so designed that they generate considerable thrust in the range from the abutment defining the prescribed thickness away from the casting and a weak thrust in the range from the abutment towards the castmg.

lf it is desired in the course of casting to regulate the prescribed contact pressure of the driving rolls, to vary the prescribed thickness of the casting or other casting parameters during the pour, the proposed method, according to yet another feature of the invention, can be performed with apparatus in which position reporting or transmitting devices for monitoring the actual thickness of the casting are provided and these are connected to a comparator, servo means being provided to control the pressure fluid for the piston-and-cylinder units in functional dependence upon the difference signal between the actual and reference thickness values appearing at the output of the comparator. Yet another advantage of such apparatus is that it permits the plant to be re-adjusted to a fresh casting program within the shortest possible stopping time between two pours.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood andobjects other than those set forth above, will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings wherein:

FIG. 1 is a side elevation of a partially shown multiple-roll withdrawing and straightening machine;

FIG. 2 is a sectional view, taken substantially along the line II-II of FIG. 3, of a different embodiment of partially shown multiple-roll withdrawing and straightening machine;

FIG. 3 is a sectional view taken substantially along the line III-III of FIG. 2;

FIG. 4 is a schematic side elevation of yet another embodiment of partially illustrated multiple-roll withdrawing and straightening machine; and

FIG. 5 is a schematic representation of a further embodiment of apparatus designed according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Describing now the drawings, FIG. 1 illustrates an arcuately bent casting 1, one part of which still contains a liquid core 9, this casting 1 being withdrawn by and straightened in a multiple-roll withdrawing and straightening machine 2. Adjustable driving rolls 3, 3', which according to their position along the part of the casting also serve to straighten the same, bear upon the upper surface of the casting 1, whereas the underside of the casting is supported by reaction rolls 4 which are mounted in fixed hearings on the foundations of the machine.

Opposed driving rolls 3, 3' and reaction rolls 4 form pairs of withdrawing or withdrawing and straightening rolls. Normally only one roll ofeach pair will be driven. If for reasons of product quality the traction or drive i.e. withdrawal force should be applied symmetrically, then both rolls of each pair may be driven. The driving rolls 3 can be displaced by suitable means, here shown for instance in the form of fluid-operated pistonandcylinder units 5, towards the cooperating reaction rolls 4 until stops 6 impact against abutment or intermediate members 7. These abutment members 7 are exchangeable to permit an adjustable variation of the gap between cooperating rolls 3 and 4 which determine the reference or prescribed thickness of the casting. Furthermore, the rolls 3 also can be adjustably moved by additional hydraulic thrustors or force generating devices 10, 10' mounted on the piston rods 5a of the piston-and-cylinder units 5, towards the casting l or towards a dummy bar 11, preferably a standard dummy bar suitable for castings having a range of different thicknesses. When a casting 1 having the desired dimensions passes between the rolls 3 and 4 the stops 6 will bear on the face of the associated abutment 7 and the pistons 12 of the thrustors 10 will be in contact with their bottom cylinder covers 13. The piston-andcylinder units 5 generate a thrust which is sufficient to prevent the casting from bulging beyond its prescribed thickness. If the casting 1 should exceed its prescribed thickness during its pass between the rolls 3, 4 of a pair, then the excess thickness will be reduced to the prescribed value by the thrust generated by the piston-andcylinder units 5. On the other hand. if the thickness of the casting I falls short of the prescribed thickness. then the contact pressure will be only that generated by the thrustors 10. This pressure is less than that generated by the thrust of the piston-and-cylinder units 5 and only sufficient for the traction or withdrawal effort a withdrawing roll in a multiple-roll withdrawing machine is required to produce. The roll 3' is in contact with a standard dummy bar 11 which is much thinner than the casting 1. It is desirable that the thrustor 10 should develop a greater hydraulic pressure than the thrustor 10 because the coefficient of friction between the roll 3' and the dummy bar 11 is smaller than that between the rolls 3 and the casting I. Shortly before or after the leading end of the casting I has advanced to the roll 3, the hydraulic pressure which is controlled by a suitable counter is diminished, to allow for the higher coefficient of friction between the casting l and the roll 3.

In the individual section 20 of a multiple-roll withdrawing and straightening machine illustrated in FIGS. 2 and 3 there are provided three upper rolls 21, two bottom driven rolls 23 and one bottom idling roll 22. The supporting idling roll 22 between the two drive rolls 23 is mounted in fixed bearings on the foundation. The upper rolls 21 are mounted in a yoke 25 which can tilt about a pivot 24. The piston-and-cylinder units 5 and the thrustor or force-applying device 10, which likewise has the form of a piston-and-cylinder unit, function in the same way as already described with reference to FIG. 1. The thrust transmitted by the yoke 25 is chosen according to the desired contact pressure of the individual rolls and the number of rolls mounted in the yoke 25. The driving rolls 23 are mounted on tiltable bearing blocks or checks 26 resting on buffer units 27. These buffer units 27 are constituted by differential hydraulic rams. Hydraulic pressure in a cylinder chamber 30 forces a piston 31 against a reference thickness abutment 32 in a position which conforms with the prescribed thickness. The same hydraulic pressure as in chamber 30 is also effective in a second cylinder chamber 33 for advancing a second piston 34 with a smaller amount of thrust. If the casting deviates from its prescribed thickness or its contour does not coincide with the theoretical pass line 35, i.e. the theoretical plane of the path of the casting, then the piston 34 will apply a weak pressure to the casting 1 beyond the pass line 35, whereas from the pass line 35 away from the casting the greater pressure generated by the thrust of piston 31 will be effective. Instead of a buffer unit 27 in the form of a differential ram a spring-loaded thrustor might also be used. According to the nature of the plant the driven rolls and the reaction rolls may be optionally supported by thrustors 10 or by buffer units 27. 7

FIG. 4 illustrates a section of a multi-roll withdrawing and straightening machine in which two consecutive rolls 41 located in the direction of travel 40 of the casting are mounted in a yoke 43. The yoke 43 can be tilted by the casting I about a pivot 42. As in the case of the yoke 25 in the arrangement of FIGS. 2 and 3 the yoke 43 is set to the casting 1 by piston-and-cylinder units 5 and thrustors 10. In order to preserve clarity in illustration in the drawing these have been conveniently omitted in FIG. 4. A pressure generating force 45 that is divided between both rolls 41 is applied to the yoke 43. The rolls 41 bear upon the casting l which has the prescribed thickness 46. Without moving the position of the pivot 42 the rolls 41 can be tilted by irregularities of the casting surface into the positions shown in chain lines indicated by reference characters 41' and 41". The roll at 41' will then be in a position corresponding to a thickness less than the prescribed thickness 46,

whereas roll 41" will be in a position corresponding to a thickness exceeding the prescribed thickness 46. If

the deformation resistance of the casting 1 is the same at both points of contact with the rolls 41' and 41 the thrust 45 acting on the yoke would allow the rolls in this position of the yoke to squeeze the casting 1 out of shape.

in order to prevent the roll in position 41 corresponding to a thickness less than the prescribed thickness 46 from squeezing the casting and from widening the dimensional tolerances of the finished casting with regard to its thickness, the invention makes the foilowing further provisions. The yoke 43 is associated at each end in the region of the outer rolls with pistonand-cylinder units 48 fitted with an inductive detector or transmitter 49. The two inductive detectors 49 continuously monitor the position of the rolls 41. Naturally any suitable instrument for measuring the position of the rolls 41 could be provided in place of inductive detectors 49. in order to permit the yoke 43 to be supported at the desired side or end in the cases that will be hereinafter described, piston-and-cylinder units 48 which oppose the pressure generating thrust 45 form supporting means that can be activated and inactivated as required. The inductive detectors or transmitters 49 and the piston-and-cylinder units are associated with a control in the form for instance of a conventional comparator, not particularly illustrated in FIG. 4 but in FIG. by unit 56 and which will be more fully considered hereinafter. The purpose of the comparator is to compare the signals received from the inductive detectors 49 with one another and each with a signal representing the prescribed casting thickness. When any one of the hereinafter described signal combinations is received the comparator activates the appropriate pistonand-cylinder unit 48. One of these signal combinations arises when one roll 41" is in a position outside that corresponding to the prescribed thickness of the casting and the other roll 41' in a position corresponding to the prescribed thickness. This signal combination causes the pressure in the piston-andcylinder unit 48 associated with the roll 41 to rise so that the contact pressure applied by this roll 41' to the casting 1 becomes less, so that the casting will not be rolledout of shape. The other signal combination will arise when for instance the roll 41 is in a position outside that corresponding to the prescribed thickness 46 by an amount 50 whereas the other roll 41' is in a position inside that corresponding to the prescribed thickness 46 by a like or an even larger amount 50. This second signal combination leads to a reduction in the pressure of roll 41', so that this roll will not unduly squeeze the casting when its thickness falls below the prescribed thickness, whereas roll 41 rolls-down the casting 1 to a little over the prescribed thickness. The remaining excess will then be rolled-down during the pass between the next pair of rolls. With these two signal combinations the contact pressure applied by one roll 41,41 is reduced to the pressure required for withdrawal, irrespective of the pressure exerted by the other roll 41".

With the described embodiments the change in contact pressure is effected by thrustors or forceapplying devices working in association with stop faces. However, as shown in the arrangement of HO. 5, the provision of stop faces can be. dispensed with and a control system may be used for controlling the contact pressure of the rolls. The pressure applied to the casting 1 by the pair of driven rolls S1, 52 may be automatically controlled by reference to a continuous measurement of the actual thickness of the casting 1. The gap width between the rolls 51, 52 and the contact pressure is adjustable by piston-and-cylinder units 53. The roll 51 is provided on one or both sides with a position reporting or transmitting device 54. Position reporting devices at each end of the rolls permit the rolls 51, 52 to be kept in precise parallelism. The roll 51 bearing on the casting 1 causes the position reporting device to generate a signal corresponding to the actual thickness of the casting 1. This signal is the input quantity for a comparator unit 56 which is also provided with a reference signal 57 representing the desired thickness. The 1 output signal and other casting parameters 59, such as the width of the casting 1, as well as the characteristic of the applied contact pressure according to the invention are applied as controlling signals to a servo unit 58 of conventional design. A pressure fluid supply 61 is also connected to the servo unit 58. The servo unit 58 controls the hydraulic fluid of the piston-and-cylinder units 53as a function of the difference signal from the comparators 56. One of the two rolls 5], 52 is driven by drive means not shown in the drawing. The bottom roll 52 may be mounted on buffers like the roll 23 in FIG. 2 or it may run in fixed bearings. For feeding the dummy bar to the mold a reference signal .57 must be provided which represents the thickness of the dummy bar and the contact pressure must be adapted to the dummy bar.

In the described example the position reporting devices 54 are associated with the roll 51. The actual thickness of the casting is thus measured directly at the pair of withdrawing rolls. However, it would also be possible to measure the actual thickness of the casting by position reporting devices located at a point which in the direction of travel of the casting preceded the pair of withdrawing rolls.

For the different above-described embodiments an example of the contact pressures required for each withdrawing and straightening roll will be hereunder given. in a multiple-roll withdrawing machine comprising six sections each fitted with three bottom rolls and three upper rolls, eleven of the bottom rolls are driven rolls. The radius of the arc of the mold and the apron is 10 meters, the roll spacing in the withdrawing machine is 540 mm. and the cross-section of the casting is 2,000 X 250 mm. The surface temperature of the casting is usually between 700C and 1,100C. Preceding the withdrawing and straightening machine the casting is guided between 36 pairs of rollers; 1n such a continuous casting machine a pressure of about 68 Mp is applied by each driving and/or straightening roll when the prescribed thickness of the casting is exceeded, whereas when the thickness drops below the prescribed thickness a pressure of about 19 Mp is necessary for withdrawing the casting. The contact pres sure for feeding the dummy bar amounts to about 33 Mp.

Under certain conditions it may be desirable to reduce the prescribed thickness of a casting progressively between consecutive pairs of rolls or in consecutive sections of a multi-roll withdrawing and straightening machine by a prescribed amount.

While there is shown and described present preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Accordingly, what is claimed is:

1. A method of operating a multiple-roll machine, especiallya multiple-roll withdrawing machine or a multiple-roll combination withdrawing and straightening machine in a continuous casting plant, during freezing of the liquid core and after the liquid core has frozen, comprising the steps of passing a casting and a dummy bar between pairs of rolls, driving at least some of said rolls, by means of each pair of rolls applying to the casting and the dummy bar as they pass therebetween, the requisite contact pressure for generating the necessary withdrawal force for the casting, and when the thickness of the casting falls below a prescribed thickness applying to the casting only a contact pressure of a magnitude required for withdrawal of the casting and when the thickness of the casting exceeds the prescribed thickness increasing the contact pressure to a greater value to again reduce the thickness of the casting to said prescribed thickness.

2. The method as defined in claim 1, further including the steps of employing at least two rolls which are interconnected and consecutively arranged in the direction of travel of the casting and which may be forced into contact with the casting and capable of tiltably responding to irregularities at the surface of the casting, comparing the actual positions of said two rolls with prescribed positions of such rolls, and upon generation ofa deviation signal owing to one such roll having been 10 pushed outwards into a position beyond that corresponding to the prescribed thickness of the casting and in the absence of a deviation signal from the other such roll reducing the contact pressure applied by said other roll to the casting irrespective of the contact pressure exerted by said one roll.

3. The method as defined in claim 1, further including the step of applying different contact pressures to the casting when its thickness falls short of the prescribed thickness and to a dummy bar which is not as thick as a casting of the prescribed thickness.

4. The method as defined in claim 1, further including the steps of employing at least two rolls which are interconnected and consecutively arranged in the direction of travel of the casting and which may be forced into contact with the casting and capable of tiltably responding to irregularities at the surface of the casting, comparing the actual positions of said two rolls with the prescribed positions of such rolls, and upon generation of a deviation signal owing to one such roll having been pushed outwards into a position beyond that corresponding to the prescribed thickness of the casting and the simultaneous generation of a deviation signal from the other such roll after having moved inwards by at least the same amount into a position'within that corresponding to the prescribed thickness, reducing the contact pressure applied by said other roll to the casting irrespective of the contact pressure exerted by said oneroll. 

1. A method of operating a multiple-roll machine, especially a multiple-roll withdrawing machine or a multiple-roll combination withdrawing and straightening machine in a continuous casting plant, during freezing of the liquid core and after the liquid core has frozen, comprising the steps of passing a casting and a dummy bar between pairs of rolls, driving at least some of said rolls, by means of each pair of rolls applying to the casting and the dummy bar as they pass therebetween, the requisite contact pressure for generating the necessary withdrawal force for the casting, and when the thickness of the casting falls below a prescribed thickness applying to the casting only a contact pressure of a magnitude required for withdrawal of the casting and when the thickness of the casting exceeds the prescribed thickness increasing the contact pressure to a greater value to again reduce the thickness of the casting to said prescribed thickness.
 2. The method as defined in claim 1, further including the steps of employing at least two rolls which are interconnected and consecutively arranged in the direction of travel of the casting and which may be forced into contact with the casting and capable of tiltably responding to irregularities at the surface of the casting, comparing the actual positions of said two rolls with prescribed positions of such rolls, and upon generation of a deviation signal owing to one such roll having been pushed outwards into a position beyond that corresponding to the prescribed thickness of the casting and in the absence of a deviation signal from the other such roll reducing the contact pressure applied by said other roll to the casting irrespective of the contact pressure exerted by said one roll.
 3. The method as defined in claim 1, further including the step of applying different contact pressures to the casting when its thickness falls short of the prescribed thickness and to a dummy bar which is not as thick as a casting of the prescribed thickness.
 4. The method as dEfined in claim 1, further including the steps of employing at least two rolls which are interconnected and consecutively arranged in the direction of travel of the casting and which may be forced into contact with the casting and capable of tiltably responding to irregularities at the surface of the casting, comparing the actual positions of said two rolls with the prescribed positions of such rolls, and upon generation of a deviation signal owing to one such roll having been pushed outwards into a position beyond that corresponding to the prescribed thickness of the casting and the simultaneous generation of a deviation signal from the other such roll after having moved inwards by at least the same amount into a position within that corresponding to the prescribed thickness, reducing the contact pressure applied by said other roll to the casting irrespective of the contact pressure exerted by said one roll. 